This invention relates to ultrasonic transducers which achieve the combination of high sensitivity and small spectral range.
It is well known that impedance matching layers on the front surface of the transducer elements, which serve as acoustic impedance matching transformers, improve the overall sensitivity of ultrasonic transducers. Two quarter wavelength front surface matching layers lead to a broad bandwidth with a very short impulse response to a delta function excitation. If the impedance of the piezoelectric transducer material is Z.sub.T and the load has impedance Z.sub.L, then for maximum sensitivity the two matching layers would have impedances ##EQU1## and thicknesses ##EQU2## where P.sub.i is the density of the i.sup.th matching layer, and f is the nominal center frequency of operation. Double quarter wave matching leads to substantial improvement in sensitivity and widening of bandwidth over the unmatched transducer.
Front surface matched phased arrays with quarter wavelength glass and plastic impedance matching layers are described in the inventors' U.S. Pat. No. 4,211,948; the body contacting wear plate in U.S. Pat No. 4,211,949; and the fabrication of such an array having an epoxy backing in U.S. Pat. No. 4,217,684.
Some signal processing applications require an ultrasonic waveform of several cycles at a single frequency. These are ultrasound systems such as Doppler instruments where only a small spectral range is desired. The patented phased array transducers are wideband devices and do not meet this requirement.